1. Field of the Invention
The present invention relates in general to the wireless telecommunications field and, in particular, to a mobile terminal containing a joint detection generalized RAKE (JD-GRAKE) receiver that utilizes a cumulative metric across the transmitting antennas which enables a reduced complexity tree search technique to be used to determine soft bit values that represent coded bits received from transmit antennas in a base station.
2. Description of Related Art
Today there is a very high level of interest in developing ways of enhancing data rates in multiple-input, multiple-output (MIMO) antenna architectures that are used in mobile communication systems which implement the high-speed-downlink-packet access (HSDPA) provision of the wideband code-division multiple-access (WCDMA) standard. For example, code-reuse (CR)-BLAST (similar to V-BLAST) and per-antenna rate-control (PARC) are two such techniques that can be used to enhance data rates in MIMO antenna systems. These two techniques are described in detail in the following two articles:                G. Foschini et al. “Simplified Processing for High Spectral Efficiency Wireless Communication Employing Multi-Element Arrays,” IEEE Journal on Selected Areas of Communications, vol. 17, pp. 1841-1852, November 1999.        S. T. Chung et al. “Approaching Eigenmode BLAST Channel Capacity Using V-BLAST with Rate and Power Feedback” Proc. IEEE VTC′02-Fall, Atlantic City, N.J., October 2001.        
The CR-BLAST and PARC techniques, as applied to the HSDPA system, both employ multi-code transmission to exploit the large capacity of the MIMO channel and thus deliver very high data rates to the mobile terminals. CR-BLAST is a spatial multiplexing technique, meaning one coded bit stream is interleaved over all transmit antennas, while PARC transmits a separately coded bit stream from each transmit antenna. Initially, the design of receivers for MIMO systems that employ such techniques often focused on the case of flat fading channels. In reality, however, the channel is often dispersive, thus causing multiple access interference (MAI) and inter-symbol interference (ISI). In addition, self-interference occurs, even in flat fading channels, since the multi-codes used in HSDPA are reused across the transmit antennas so as to avoid a code limitation problem.
Since the memory and/or processing power of mobile terminals is typically quite limited, the challenge in receiver design for the dispersive MIMO scenario is to achieve a good balance between performance and complexity in a receiver. This is especially true since the number of signals the receiver needs to demodulate is large due to multi-code and multi-antenna transmission. On one extreme of the complexity scale is the conventional RAKE receiver that performs poorly since RAKE is designed for white noise, and the ISI and MAI is colored. Moreover, the conventional RAKE receiver fails due to self-interference from code reuse. The other extreme is occupied by full joint-demodulation receivers, which perform very well, but are extremely complex. Somewhere in the middle, for example, are receivers like minimum-mean-squared (MMSE)-GRAKE receivers that employ some form of either linear or decision feedback equalization. A detailed description about the MMSE-GRAKE receiver is provided in the following article:                S. J. Grant et al. “Generalized RAKE Receivers for MIMO Systems” in Proc. VTC′03-Fall, Orlando, Fla., October 2003.        
Although the MMSE-GRAKE receiver works well in frequency selective fading, it underperforms severely in lightly dispersive or nearly flat scenarios. As such, the Joint-Detection (JD)-Generalized RAKE receiver (JD-GRAKE receiver) was recently developed to restore performance in such cases, and also is described by Grant et al. in the above referenced article. The JD-GRAKE receiver can also be used in MIMO configurations where the number of receive antennas is less than the number of transmit antennas. In these cases, the JD-GRAKE receiver outperforms the MMSE-GRAKE receiver in all levels of dispersion.
The JD-GRAKE receiver is able to handle the various types of interference by forming a partition of signals reminiscent of group detection for CDMA. Specifically, subsets of signals that share the same channelization code are formed and joint detection is applied to the M signals within each subset, where M is the number of transmitting antennas at the base station. This resolves the interference due to code reuse. The ISI and MAI from signals outside each subset are suppressed in a similar fashion as in the conventional single-antenna GRAKE receiver. That is, the ISI and MAI are treated as colored Gaussian noise and the correlation of the interference across fingers on multiple receive antennas is exploited in adapting the finger delays and combining weights. This detection procedure is performed separately for each of the K channelization codes.
A problem with the JD-GRAKE receiver, however, is that when higher-order modulation is used in conjunction with a large number of transmit antennas, the number of metrics to compute in forming the soft bit values required by a decoder becomes very large. Specifically, with M transmit antennas and a signal constellation containing Q points, the number of metrics to compute per symbol period is QM. For example, with 16-QAM (Q=16) and M=4 transmit antennas, the number of metrics is 65,536—a large number indeed. One way to address this problem and avoid the exponential complexity in the JD-GRAKE receiver is to use a receiver based on successive cancellation as described in U.S. patent application Ser. No. 10/795,101 entitled “Successive Interference Cancellation in a Generalized RAKE Receiver Architecture” and filed on May 5, 2004, which is incorporated by reference herein. The exponential complexity is avoided in the successive cancellation receiver by successively detecting the M transmitted signals in a multi-stage approach. The present invention, meanwhile, addresses complexity growth by introducing a technique that significantly reduces the number of metric computations of the JD-GRAKE receiver, allowing near-optimal joint detection to be performed in a single stage.